To date, a nonwoven fabric material or filter paper material has been used for an air filter of an automobile engine, and these materials are pleated to increase a filter medium area in a unit volume, thereby obtaining a predetermined dust holding capacity. For example, in a case where a nonwoven fabric material is used, although the number of pleats is small, the thickness of a filter medium is increased, and deep filtration can be thus performed, thereby obtaining a predetermined dust holding capacity. Meanwhile, in a case where a filter paper material is used, the number of pleats is increased and a filter medium area is thus sufficiently increased, thereby obtaining a predetermined dust holding capacity. In order to further increase the dust holding capacity, a pre-air-filter formed from a nonwoven fabric may be provided upstream of such an air filter (main filter).
More specifically, an air filter for an engine is required to have properties such as reduced pressure loss, enhanced dust cleaning efficiency, and increased dust holding capacity. In particular, in recent years, enhanced dust cleaning efficiency is highly required. In order to achieve enhanced dust cleaning efficiency, it is effective to form a fine mesh of the filter material. However, a drawback in that clogging is accelerated may occur. To date, a pre-air-filter for obtaining a dust holding capacity has been accompanyingly provided preceding a main-air-filter for an engine in order to reduce the clogging and reduce filter replacement frequency. If the pre-air-filter is clogged soon, an intended purpose of reducing the filter replacement frequency cannot be achieved. Therefore, the pre-air-filter is designed so as to have a coarse mesh. Further, the pre-air-filter is designed to be thick in order to increase the dust holding capacity. Further, such a pre-air-filter needs to be designed to be formed from such a material that the pre-air-filter is prevented from being deformed in the thickness direction under a load due to dust and becoming thin, and pressure loss of the pre-air filter is less likely to occur. To date, as such a pre-air-filter, a resin bond type short-fiber nonwoven fabric obtained by bonding fiber webs by a sprayed resin binder has been used because the thickness is likely to be assured (for example, Patent Literature 1).